Much interest has been placed today on water-based paints for reducing the emission of VOC to the environment. Air drying water-based paints of emulsion type are now in wide use. These emulsion type paints rely, as their film-forming mechanism, on physical cohesion or physical fusion of emulsion particles upon evaporation of water. However, the performance of their films such as water-resistant, solvent-resistant and anti-chemical properties is not satisfactory compared to the films produced from thermosetting coating compositions. Room-temperature curable coating compositions containing an external crosslinker such as polyisocyanate crosslinkers must be provided as a two-part composition particularly when high crosslinking densities are desired. This is because once mixed the composition can no longer be stored for a long period of time.
A reaction known as the Michael reaction in which an activated methylene group is added to a polarized carbon-to-carbon double bond such as .alpha.,.beta.-unsaturated carbonyl has been employed as a crosslinking mechanism of coating compositions. These compositions are advantageous in that the crosslinking reaction proceeds without forming any reaction by-product and newly formed crosslinking bonds are chemically stable carbon-to-carbon bonds. These type of compositions are also not stable upon long-term storage when formulated in a one-pot composition. This is particularly true when the composition contains a base which catalyzes the Michael reaction. Conversely, if the compositions could be formulated in a storage-stable, one-pot composition, they would require an elevated temperature at least above 100.degree. C. or a long period of time for baking. For example, U.S. Pat. No. 4,408,018 discloses a room-temperature curable composition containing an acrylic polymer having a plurality of pendant acetoacetate groups, a crosslinker such as trimethylolpropane triacrylate and a basic catalyst. This composition is not stable upon long term storage when all reactants are mixed together. JP-A-05247372 (Laid Open No. 247372/93) discloses a one-pot composition containing a binder resin having a plurality of
.alpha., .beta.-unsaturated carbonyl groups and an activated methylene crosslinker. This composition does not contain a catalyst and is stable upon storage. However, its baking requires an elevated temperature higher than 100.degree. C. EP-A-596461 discloses a water-based coating composition utilizing as its curing mechanism a Michael reaction. Although it is taught that the composition may be cured at a relatively low temperature, for example, at 60.degree. C., at least one of the Michael reactants or catalyst must be kept separate from the remainder of the composition until immediately before use.
A crosslinkable coating composition is known which utilizes a reaction between carbonyl and hydrazide groups. For example, JP-A-02155956 (Laid Open No. 155956/90) discloses a coating composition comprising an emulsion of an acrylic copolymer having a plurality of carbonyl groups and a polyhydrazide crosslinker. This composition also has a limited storage stability particularly when a high crosslinking density is desired, and film properties, particularly acid resistance are not satisfactory because the newly formed crosslinking bond is a Schiff base type carbon-to-nitrogen bond.
A need exists, therefore, for a one-pot water-based coating composition which cures through a Michael reaction at room temperature yet retaining a satisfactory storage stability and gives a film having excellent properties.